An EPS Mobility Management (EMM) attach procedure is typically used for a user equipment (UE) to attach to an Evolved Packet Core Network (EPC) for packet services in Evolved Packet System (EPS). With a successful attach procedure, a context is established for a UE in an Mobility Management entity (MME), and a default bearer would be established between the UE and a Packet Data Network Gateway (PDN GW) and thus enabling an always-on IP connectivity to the UE.
An attach attempt counter would be used to limit the number of subsequently rejected attach attempts. For a long term evolution (LTE) communication system for example, the attach attempt counter would be incremented as specified in subclause 5.5.1.2.6 of the 3GPP TS 24.301 specification (version v11.4.0 as on September 2012). Depending on the value of the attach attempt counter, specific actions would be performed.
According to the above mentioned specification, the attach attempt counter would be reset under circumstances when the UE is powered on, a USIM is inserted, an attach or combined attach procedure is successfully completed, a GPRS attach or combined GPRS attach procedure is successfully completed in A/Gb or Iu mode, a combined attach procedure is completed for EPS services only with cause #2, #16, #17, #18 or #22, an attach or combined attach procedure is rejected with cause #11, #12, #13, #14, #15 or #25, or a network initiated detach procedure is completed with cause #11, #12, #13, #14, #15 or #25. Additionally the attach attempt counter shall be reset when the UE is in substate EMM-DEREGISTERED.ATTEMPTING-TO-ATTACH and when a new tracking area is entered, the timer T3402 has expired, and timer T3346 has started.
The followings will describe how the attach attempt counter would be incremented. Typically, the attach attempt counter would be not used when a UE successfully attaches to a network but would be used to count the number of re-tries under the circumstances when a UE experiences attachment failures to a core network. FIG. 1 illustrates a typical EMM attachment procedure according to a current version of the LTE specification. In step S101, a UE would attach to a network by first transmitting an ATTACH REQUEST message and start the timer T3410. If the attach request is accepted by the network, the MME would send an ATTACH ACCEPT message to the UE and start timer T3450. The MME would then send the ATTACH ACCEPT message together with an ACTIVATE DEFAULT EPS BEARER CONTEXT REQUEST message contained in the EPS Session Management (ESM) message container information element to activate the default bearer as described in subclause 6.4.1. A network may also initiate the activation of dedicated bearers towards the UE by invoking the dedicated EPS bearer context activation procedure as described in subclause 6.4.2. Upon receiving the ATTACH ACCEPT message, the UE would stop timer T3410.
If the attach request cannot be accepted by the network, in step S102, the MME would send an ATTACH REJECT message to the UE including an appropriate EMM cause value. If the attach procedure fails due to a default EPS bearer setup failure, an ESM procedure failure, or operator determined barring is applied on default EPS bearer context activation during attach procedure, the MME shall combine the ATTACH REJECT message with a PDN CONNECTIVITY REJECT message contained in the ESM message container information element. In this case the EMM cause value in the ATTACH REJECT message would be set to #19 “ESM failure”.
Upon receiving the ATTACH REJECT message, in step S103, the UE would stop the timer T3410 and determined the received EMM cause value in order to determine a proper action based on the EMM cause value. At this point, the UE may take actions based on various EMM cause values as indicated in clause 5.5.1.2.5 as in step S104. For abnormal causes as specified in 5.5.1.2.6, the UE may subsequently increment the attach attempt counter.
For example, under the circumstance such as case (d) in subclause 5.5.1.2.6 as in step S105, after ATTACH REJECT has been received by the UE for EMM cause values other than those mentioned in subclause 5.5.1.2.5 and cases of EMM cause #22 if considered as abnormally cases according to subclause 5.5.1.2.5, upon reception of the EMM cause #19 “ESM failure”, if the UE is not configured for NAS signalling low priority, the UE may set the attach attempt counter to 5. Upon reception of the EMM causes #95, #96, #97, #99 and #111 the UE would set the attach attempt counter to 5.
Therefore in step S106, a UE would increment the attach attempt counter when it fails to attach to the MME under the circumstance as described in subclause 5.5.1.2.6. This means that for some reject causes such as network error, the device would not only fail to attach but also fail for 5 times as the attempt counter default value is set to five. This means that even though the network would take some time to recover from an error, a UE would nevertheless still try to attach to a MME and send an attachment request five times. This would cost unnecessary signaling and power consumption. Therefore, a method for setting the attach attempt counter value by the network would need to be proposed.